1. Field of the Invention
The present invention relates to a retardation optical element for use in a liquid crystal display or the like, especially a retardation optical element that includes a retardation layer having a cholesteric-regular molecular structure and can compensate for the state of polarization of light that slantingly emerges from a liquid crystal cell in the direction deviating from its normal, to a method of producing the retardation optical element, and to a polarization element and a liquid crystal display, each including the retardation optical element.
2. Description of Related Art
FIG. 13 is a diagrammatic exploded perspective view of a conventional, general liquid crystal display.
As shown in FIG. 13, the conventional liquid crystal display 100 includes a polarizer 102A on the incident side, a polarizer 102B on the emergent side, and a liquid crystal cell 104.
Of these component parts, the polarizers 102A and 102B are so constructed that they selectively transmit only linearly polarized light having the plane of vibration in a predetermined direction, and are arranged in the cross nicol disposition so that the direction of vibration of the linearly polarized light transmitted by the polarizer 102A is perpendicular to that of vibration of the linearly polarized light transmitted by the polarizer 102B. The liquid crystal cell 104 include a large number of cells corresponding to pixels, and is placed between the polarizers 102A and 102B.
A case where the liquid crystal cell 104 in the above-described liquid crystal display 100 is of VA (Vertical Alignment) mode, which a nematic liquid crystal having negative dielectric anisotropy is sealed in a liquid crystal cell, is now taken as an example. Linearly polarized light that has passed through the polarizer 102A on the incident side passes, without undergoing phase shift, through those cells in the liquid crystal cell 104 that are in the non-driven state, and is blocked by the polarizer 102B on the emergent side. On the contrary, the linearly polarized light undergoes phase shift when it passes through those cells in the liquid crystal cell 104 that are in the driven state, and the light in an amount corresponding to the amount of this phase shift passes through and emerges from the polarizer 102B on the emergent side. It is therefore possible to display the desired image on the polarizer 102B side (i.e., on the emergent side by properly controlling the driving voltage that is applied to each cell in the liquid crystal cell 104. The liquid crystal display 100 is not limited to the above embodiment in which light is transmitted and blocked in the above-described manner, and there is also a liquid crystal display so constructed that light emerging from those cells in the liquid crystal cell 104 that are in the non-driven state passes through and emerges from the polarizer 102B on the emergent side, and that light emerging from those cells that are in the driven state is blocked by the polarizer 102B on the emergent side.
Discussion is now made on a case where linearly polarized light passes through the non-driven-state cells in the above-described liquid crystal cell 104 of VA mode. The liquid crystal cell 104 is birefringent, and its refractive index in the direction of thickness and that in the direction of plane are different from each other. Therefore, of the linearly polarized light that has passed through the polarizer 102A on the incident side, the light that has entered the liquid crystal cell 104 along its normal passes through the liquid crystal cell 104 without undergoing phase shift, but the light that has slantingly entered the liquid crystal cell 104 in the direction deviating from its normal undergoes phase shift while it passes through the liquid crystal cell 104, and becomes elliptically polarized light. The cause of this phenomenon is that those liquid crystalline molecules that are vertically aligned in the liquid crystal cell 104 when the cells in the liquid crystal cell 104 of VA mode are in the non-driven state function as a positive C plate. It is noted that the amount of phase shift that occurs for light passing through the liquid crystal cell 104 (transmitted light) is affected also by the birefringence of the liquid crystalline molecules sealed in the liquid crystal cell 104, the thickness of the liquid crystal cell 104, the wavelength of the transmitted light, and so on.
Owing to the above-described phenomenon, even when the cells in the liquid crystal cell 104 are in the non-driven state and linearly polarized light is supposed to be transmitted through the liquid crystal cell 104 as it is and blocked by the polarizer 102B on the emergent side, a part of the light that emerges slantingly from the liquid crystal cell 104 in the direction deviating from its normal is to leak from the polarizer 102B on the emergent side.
For this reason, the above-described conventional liquid crystal display 100 has the problem (so-called viewing angle dependency problem) that the display quality at the time when an image is viewed slantingly from a position not on the normal of the liquid crystal cell 104 is lower than that at the time when the image is viewed from the front of the display.
To eliminate the viewing angle dependency problem of the aforementioned conventional liquid crystal display 100, there have been developed a variety of techniques up to now. One of them is the liquid crystal display described, for example, in Patent Document 1 (Japanese Laid-Open Patent Publication No. 67219/1991). This liquid crystal display uses a retardation optical element including a retardation layer having a cholesteric-regular molecular structure (a retardation layer having double refractivity), where the retardation optical element is placed between a liquid crystal cell and a polarizer in order to provide optical compensation.
In the retardation optical element having a cholesteric-regular molecular structure, the selective reflection wavelength given by the equation λ=nav·p (p: the helical pitch in the helical structure consisting of liquid crystalline molecules, nav: the mean refractive index of a plane perpendicular to the helical axis), is so adjusted that it is either shorter or longer than the wavelength of transmitted light, as described in Patent Document 2 (Japanese Laid-Open Patent Publication No. 322223/1992), for example.
In the aforementioned retardation optical element, linearly polarized light that has slantingly entered the retardation layer in the direction deviating from its normal undergoes phase shift, while passing through the retardation layer, to become elliptically polarized light, like in the case of the above-described liquid crystal cell. The cause of this phenomenon is that the cholesteric-regular molecular structure functions as a negative C plate. The amount of phase shift that occurs for light passing through the retardation layer (transmitted light) is affected also by the birefringence of the liquid crystalline molecules in the retardation layer, the thickness of the retardation layer, the wavelength of the transmitted light, and so on.
Therefore, the viewing angle dependency problem of conventional liquid crystal displays can successfully be solved by the use of the above-described retardation optical element if the retardation layer contained in the retardation optical element is properly designed so that the phase shift that occurs in a liquid crystal cell of VA mode, which functions as a positive C plate, and the phase shift that occurs in the retardation layer contained in the retardation optical element, which functions as a negative C plate, are canceled each other.
However, it has been found that the viewing angle dependency problem can be solved if the above-described retardation optical element (a retardation layer having a cholesteric-regular molecular structure) is placed between a liquid crystal cell and a polarizer, but that, when the retardation optical element is so provided, bright and dark fringes could appear on a displayed image to drastically lower the display quality.
The inventor has made earnest studies to find the causes of this phenomenon by conducting experiments and computer-aided simulations, and, as a result, finally found that one of the causes is the directions of the directors of liquid crystalline molecules on the surfaces of the retardation layer contained in the retardation optical element.